[libcxx] r285382 - Add __libcpp_version file and __libcpp_library_version function.

[Joerg Sonnenberger via cfe-commits]

On Sun, Oct 30, 2016 at 01:49:53PM -0600, Eric Fiselier via cfe-commits wrote:
> >  E.g. presence of libc++ won't tell you if you can use sized deallocation
> > as that's a ABI library issue.
> 
>  The value of __libcpp_version could easily be updated by vendors to store
> the version of the system dylib,
> so I don't see why this wouldn't work.

As I said, this is not a decision based on the libc++ version. When we
start adding random extra flags to the version file, it makes the point
I'm raising even bigger: we are duplicating data and clang should not be
magically changing its behavior like that anyway.

Joerg